In ultra high speed transmission systems having a bitrate of a signal per wavelength of 100 Gbit/s or more, digital coherent technology that combines a coherent optical communication technology and a digital signal processing technology has been widely used. The DP-QPSK (Dual Polarization-Quadrature Phase Shift Keying) system described in NPL 1 is a standard system as a modulation/demodulation system in 100 Gbit/s long-distance optical transmission systems. In 100 Gbit/s long-distance optical transmission systems, for example, 4-value phase modulation is used to generate a 32 Gbit/s signal, this signal is doubly multiplexed to generate a coherent optical signal, and the coherent optical signal is further doubly multiplexed using two polarized waves to generate a 128 Gbit/s coherent optical signal. By wavelength-multiplexing DP-QPSK optical signals of different wavelengths, it is possible to achieve an optical transmission system having a transmission capacity of several terabits per second.
The reception side performs coherent detection using local oscillator having the same wavelength as the signal light, digitizes the received signal using an AD converter, and performs digital signal processing using a DSP. This performs the chromatic dispersion compensation and polarization mode dispersion compensation of the transmission line, the demodulation and phase estimation of polarization multiplexed signals, and the like, thereby achieving excellent transmission characteristics.
On the other hand, in the field of radio transmission, NPL 2 proposes, as an approach for further improving reception characteristics, the MIMO diversity technology for improving receiver sensitivities using a plurality of receivers.